In an internal combustion engine that employs V-type cylinder arrangement, a separate exhaust purifying catalyst is provided per bank having a group of cylinders. In such an internal combustion engine, if the bed temperatures of the exhaust purifying catalysts vary from one another, the purifying performance undesirably varies per the exhaust purifying catalyst. Therefore, it is difficult to appropriately perform an exhaust purifying control. That is, even if the exhaust purifying control is adjusted to a certain exhaust purifying catalyst, the exhaust purifying control is not suitable for another exhaust purifying catalyst having different bed temperature. Thus, the exhaust gas may not be sufficiently purified.
In the prior art, an exhaust purifying apparatus for a V-type internal combustion engine, which includes a separate exhaust purifying catalyst per bank, has been proposed. The exhaust purifying apparatus detects the bed temperature of each exhaust purifying catalyst and adjusts the ignition timing per bank through a feedback control (For example, Japanese Laid-Open Patent Publication No. 11-117786). The exhaust purifying apparatus reduces the variation of the catalyst bed temperatures by decreasing the temperature of exhaust gas by delaying the ignition timing of the bank having a higher catalyst bed temperature and increasing the temperature of exhaust gas by advancing the ignition timing of the bank having a lower catalyst bed temperature.
The variation of the catalyst bed temperatures is the main cause of variation of heat received by the exhaust purifying catalyst from exhaust gas. If the state of heat reception and heat radiation of the exhaust purifying catalyst are maintained constant, the variation in the amount of heat received by each exhaust purifying catalyst from exhaust gas is automatically corrected by equalizing the catalyst bed temperatures of the exhaust purifying catalysts. However, in the internal combustion engine of, for example, a vehicle, the temperature and the flow rate of exhaust gas change by a large amount when the operating condition is changed. Accordingly, the amount of heat received by the exhaust purifying catalyst fluctuates by a large amount. Therefore, even if the bed temperatures of the exhaust purifying catalysts temporarily become equal to one another, the amount of heat received by each exhaust purifying catalyst is not necessarily equal to that of the other catalyst. Thus, it is difficult to maintain the bed temperatures of the exhaust purifying catalysts to be equal to one another. Therefore, when the ignition timing is adjusted only based on the bed temperature of each exhaust purifying catalyst, delay and excess response in feedback adjustment or control hunting may occur.